2.3. Top Quark Physics
2.3.3. Top Couplings: Top & Co
The top quark couples to the SM fields through its gauge and Yukawa interactions. Some of these couplings have been investigated at the Tevatron, through studies of the W tbvertex and the ttγ¯ production, while others, such as the t¯tZ and t¯tH production, are becoming accessible
13Theτ mean lifetime is of the order of 10−13s [4].
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2.3. Top Quark Physics
only with the high statistics top quark sample at the LHC, also called for this reason a ‘top quark factory’. At hadron colliders, the first evidence of the coupling of the top quark to the γ, Z,and H boson will come from the production rate, while constraints on the coupling of the top quark with theW-boson come from both the top quark decay and the single top production.
New physics related to EWSB may be found first in top quark precision measurements. Pos-sible new physics signals would cause deviations of the top quark couplings tZ, tγ, and W tb, from the SM prediction. Some examples include technicolor and other models with a strongly coupled Higgs sector [58].
Figure 2.9 shows a summary of the processes involving the top quark at hadron colliders that provide information of the coupling of the top quark with the corresponding bosons at the LHC.
Figure 2.9.: Summary of the different processes involving the top quark coupling to theZ, W, γ, H and g bosons.
Top & W
Experimental information on the coupling of the top quark to the W-boson can be obtained from the top quark decay and from electroweak single top quark production.
• Top Decay: W polarisation
Since the top quark decays almost exclusively to W+b, the measurement of the W-boson helicity in top quark decays probes the structure of theW tbvertex, which in the SM is V-A.
Since theW-bosons are produced as real particles in top quark decays, their polarisation can be longitudinal, left-handed or right-handed. The fractions with a certain polarisation, F0, FL and FR, can be extracted from measurements of the angular distribution of the decay products of the top quark, given by:
1 σ
dσ
dcosθ∗ = 3
4(1−cos2θ∗)F0+3
8(1−cosθ∗)2FL+3
8(1 + cosθ∗)2FR, (2.33) where θ∗ is defined as the angle between the W-boson momentum in the top quark rest frame and the momentum of the down-type decay fermion in the rest frame of theW-boson.
2. Physics
The next-to-next-to-leading-order (NNLO) QCD prediction for the helicity fractions in the SM, for a top quark mass mt = 172.8 GeV and a b-quark mass mb = 4.8 GeV, are F0 = 0.687±0.005, FL = 0.311±0.005 and FR = 0.0017±0.0001 [59]. Recent measurements of the W-boson helicity fractions have been performed by both CDF and D0 experiments at the Tevatron [60, 61, 62] and by ATLAS and CMS experiments at the LHC [63, 64, 65, 66]. All measurements are in agreement with SM predictions.
• Top Decay: Rb
Under the assumption of a unitary 3×3 CKM matrix, the top quark decays almost exclu-sively to W b(|Vtb| ≈ 1). A fourth generation of quarks would accommodate significantly smaller values of|Vtb|, affecting, for example, the decay rates in thett¯production channel.
Therefore, a measurement of the ratio of branching fractions of the form:
R= B(t→W b)
B(t→W q) = |Vtb|2
|Vtb|2+|Vts|2+|Vtd|2, (2.34) would test the three generations assumption. A measurement of|Vtb|can also be extracted from R by assuming a unitary 3×3 CKM matrix, where R = |Vtb|2. The most precise measurement to date ofRhas been performed by the CMS Collaboration at√
s= 8 TeV, resulting in an unconstrained measured value of R = 1.014±0.003 (stat.)±0.032 (syst.), which translates into |Vtb|= 1.007±0.016 (stat.+syst.) under the three-generation CKM matrix assumption, and a lower limit of|Vtb|>0.975 at 95% CL when requiring|Vtb| ≤1, all consistent with SM predictions [67].
• Single Top Production
As seen in Figure 2.5, all of the single top quark channels include an interaction between a top quark, a bottom quark, and a W-boson. The strength of this W tb interaction is given by the CKM matrix element Vtb. Observations of single top quark events can thus provide direct measurements of Vtb without assuming unitarity, and, at the same time, test for additional structure in the CKM matrix. The most precise measurement to date of Vtb has been performed by the CMS experiment, combining the single top quark mea-surements in the t-channel at √
s= 7 and 8 TeV. In the approximation |Vtd|,|Vts| |Vtb| and parametrisation of a possible anomalous form factor that could modify the coupling strength as fLv, theVtb matrix element can be obtained as:
|fLvVtb|=
sσt−ch.
σt−ch.theo.. (2.35)
This relationship yields a combined measured value of: |fLvVtb|= 0.998±0.038 (exp.)± 0.016 (theo.) [68].
Top & γ
At hadron colliders, a measurement of the tγ coupling via qq¯→γ∗ →tt¯is unrealistic due to the overwhelming contribution from the QCD processesqq¯→g∗ →t¯t and gg→ t¯t. Therefore, a more feasible approach to probe the tγ coupling is via the measurement of the associated production of a photon with a top quark pair. The photon can be radiated from:
• the top quark: pp→t¯tγ, with the top quark decaying without photon radiation, or
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2.3. Top Quark Physics
• the top quark decay products: pp → t¯t, with a photon radiated from the decay of an on-shell top quark (t→W bγ).
Only events of the first type are sensitive to the tγ coupling, and therefore, to the top quark electric charge. However, for a well-definedt¯tγ final state, all interferences between both types of processes have to be taken into account. First evidence of the associated production of photon radiation and a top quark pair was announced by the CDF Collaboration at√
s= 1.96 TeV [69], followed by measurements by the ATLAS Collaboration and CMS Collaboration at√
s= 7 TeV and 8 TeV, respectively [70, 71]. All measurements are in agreement with SM predictions.
Top & H
One of the important tests of the SM is the measurement of the top quark Yukawa coupling.
The coupling of the top quark to the Higgs can be studied from the production rates of the Higgs boson in pp collisions at the LHC. Since gluon-gluon fusion production of the Higgs boson proceeds via loop contributions, the heaviest particles are expected to contribute the most. Therefore, indirect constraints on the top Yukawa coupling can be made from gluon-gluon fusion production of the Higgs boson, as well as from H → γγ decays, where the same loop contributions occur. This, however, assumes no additional heavy particles which could couple to the Higgs boson. The only direct measurement of the top Yukawa coupling at the LHC can be performed in thettH¯ andtH channels, corresponding to the associated production of a top quark pair with a Higgs boson or the production of a single top quark and the Higgs boson, respectively.
The production of the Higgs boson in association with a single top quark, tH, is strongly suppressed with respect tot¯tH production, due to the substantial cancellation between the two diagrams where the Higgs boson is emitted from the top quark or from theW-boson exchanged in the t-channel. Since the resulting cross section is very small, any non-standard physics affecting the cancellation (e.g. change of the sign of the tH coupling) will lead to a much larger cross section, making this process an interesting window to search for new physics.
The measurements of thet¯tHfinal state are not trivial, since not only it is the Higgs production mechanism with the smallest cross section, but also its signature is quite complicated.
Searches have been performed by both ATLAS and CMS Collaborations in the following channels:
• t¯tH, H → γγ, with very small branching ratio (0.2%), with QCD multi-photon/jet final states as main backgrounds. The Higgs boson can be reconstructed in this case as a narrow mass peak.
• t¯tH, H → W W/ZZ, with significant branching ratio (22% for H → W W), with main background contribution fromt¯tZ and t¯tW processes, as well as from processes with non-prompt leptons. Leptonic decays of the W- and Z-bosons can give a distinct signature with multiple leptons, which challenges the reconstruction of the Higgs boson.
• t¯tH, H→b¯b, which gives the largest branching ratio of allt¯tHchannels (58%), but needs a good understanding of its large main irreducible background,t¯tproduction in association with extra jets (typically heavy flavour jets). Since the final state involves multiple b-quarks, the reconstruction of the Higgs boson as a peak in the invariant mass spectrum of two b-jets becomes challenging.
2. Physics
Neither experiment observes a significant ttH¯ signal so far. The latest limits that have been set by the ATLAS experiment are: in the t¯tH, H →b¯bfinal state at √
s= 8 TeV, the observed (expected) limit for a Higgs boson mass of 125 GeV is 4.1 (2.6) times the SM cross section at 95% CL [72], and in the t¯tH, H → γγ final state at both √
s = 7 and 8 TeV combined, the observed (expected) limit is 6.7 (4.9) times the SM cross section times the branching ratio BR(H → γγ) at mH = 125.4 GeV at 95% CL [73]. The CMS experiment combined their results in all three aforementionedt¯tH final states, yielding a combined best-fit signal strength, µ= σt¯tH/σtSM¯tH, assuming an mH = 125 GeV, of µ = 2.9+1.1−0.9. This result corresponds to a 3.5 standard deviation excess over the background-only hypothesis, and represents a 2.1 standard deviation upward fluctuation with respect to the SMttH¯ expectation. This excess is driven by thet¯tH, H →W W/ZZ channel with two leptons with same-sign charge in the final state, with a best-fit value of µ= 5.3+2.1−1.8 [74].
Further information and references to theoretical calculations of this process can be found in [56].
Top & Z
Similarly to thetγcoupling measurement, a measurement of thetZcoupling viaqq¯→Z∗ →tt¯ at hadron colliders is hard given the dominance of QCD processes. Instead, the neutral current coupling of the top quark can be directly measured via the associated production of aZ-boson and a top quark pair, t¯tZ. This process, together with ttW¯ , constitute the main topics of this thesis and will be explained in more detail in the following section.